The control chart: an epidemiological tool for public health monitoring.

The objective of the authors is to apply the control chart, a statistical method for quality control used in industry, to public health surveillance. A pilot study was conducted during the 1998 World Football Cup (WFC) by 553 sentinel general practitioners (GPs) throughout France. The average number of cases of communicable, environmental and societal diseases relating to mass gatherings, and the total number of referrals to hospital reported daily by a GP, were plotted on a u-chart for each condition monitored. This average was beyond the statistical control limits if it fell outside the 99.7% confidence interval of the baseline incidences estimated before the WFC. Seven hundred and forty data points representing 262 279 medical encounters were plotted. Nineteen points exceeded the statistical control limits. None of these alerts was confirmed for two consecutive days. Control charts ensured that the level of the items chosen for general community health surveillance remained under control.